The invention relates to high speed data communication on band-limited channels, and more particularly to the transmission of data at 14,400 bits per second on voice band channels having a passband of approximately 300 to 3000 Hertz.
Modulation techniques which have evolved for obtaining higher data rates over telephone voice channels of 300 to 3000 Hertz bandwidth generally rely upon quadrature amplitude modulation (QAM). In this technique, two carrier waves, at the same frequency but 90.degree. out of phase are independently modulated, and at the receiver are independently demodulated. Such a scheme is well-known and is described in detail in "Information Transmission, Modulation and Noise", by Mischa Schwartz, Third Edition, McGraw Hill Book Co., pages 226-235. This reference discloses a 4 by 4 QAM state space or constellation which is widely used in the United States for 9600 bit per second transmission over voice channels. Also disclosed by this reference is a technique which is used for both domestic and foreign 9600 bit per second transmission over voice channels. This latter system is described in U.S. Pat. No. 3,887,768 and its associated constellation is shown in FIG. 3B of this patent. These 16 state systems reduce bandwidth by taking groups of 4 bits and assigning them to each of the 16 states. In general the total bandwidth required by such a system is ##EQU1## where N=Bits/Baud
Thus the above 9600 bit per second system requires 2400 Hz minimum bandwidth. Other constellations for high speed modems are also disclosed in U.S. Pat. No. 3,983,485.
For data rates above 9600 bits per second, constellations of more than 16 states are normally required to restrict bandwidth to the limits of a voice channel. For example, an 8 by 8 QAM system (64 states) operating at 14,400 bits per second encodes 6 bits per baud, so from the preceding equation the total bandwidth required is 2400 Hertz (the same as the 9600 bit per second systems previously described). This allows 14,400 bits per second (BPS) transmission through a voice channel. Due to the closer packing of states in a 8 by 8 QAM system, the error rate for a given noise level will be poorer than the 4 by 4 QAM previously cited. See also U.S. Pat. No. 4,271,527 which shows two slightly modified 4 by 4 QAM systems for 14,400 BPS transmission.
In U.S. Pat. No. 4,123,710 a modulation technique employing two six level baseband systems which are partial response encoded to produce two eleven level systems are discussed. The signals then modulate two carriers in phase quadrature to produce an 11 by 11 QAM signal constellation.